In at least UK populations, and perhaps others, SNP rs2187668 is a tag SNP for the HLA-DRB1*0301 allele.

The HLA-DRB1*0301 allele is the allele presenting the highest risk for developing lupus, and it appears to act in a dominant manner (i.e. inheriting 2 copies is no worse than inheriting 1 copy). In dbSNP orientation, the risk allele is rs2187668(A), with an odds ratio of 2.3x (CI: 1.7 - 3.2, permuted p < 0.0001). [PMID 17997607]

From individuals with the most common rs2187668(G;G) genotype, risk is reduced for celiac disease, with an odds ratio of 0.30x according to a study of ~800 patients.[PMID 17558408]

rs2187668(A) also tags the tightly linked DQB1*0201 allele, which in turn is linked to DQA1*0501; together, these are known as the DQ2.5 haplotype. This is the most common haplotype associated with celiac disease in Europeans.[PMID 18509540]